Generally known automatic-control valve devices prevalently used for controlling pressure or flow rate sense the pressure of a fluid flowing in the associated pipeline, and operate a main valve driving piston by a pilot valve operated by controlling the flow rate of the fluid by a fixed restriction control device, such as a needle valve. Those generally known automatic-control valve devices, however, have problems in the durability of a sealing mechanism for sealing the main valve driving piston and in the machining accuracy of the component parts of the sealing mechanism. Those generally known automatic-control valve devices need a fixed restricting device, such as a needle valve, in a passage for supplying a fluid to apply a pressure at a position on the upstream side of the main valve to the main valve driving piston to drive the main valve gradually in order that pressure pulsation due to the sudden change of the flow of the fluid can be avoided. The fixed restricting device, which has a minute, precision construction, is liable to be clogged with sand or dust.
The automatic-control lift valve of the original invention disclosed in JP-B-8-6837 solves those problems, is provided with a pilot valve unit not provided with any fixed restricting device, such as a needle valve, and free from clogging, and a main valve unit capable of being easily and simply sealed, capable of holding pressure or flow rate in a passage on the downstream side of the main valve unit at a predetermined value, and having a simple construction.
The gist of the construction of this prior art automatic-control lift valve device mentioned in the specification are itemized as follows.
(1) In the automatic-control lift valve device as an automatic constant-pressure valve as shown in FIG. 9, a main valve unit having a main valve casing 1 has a main valve element 5 and a main valve driving member 6 formed integrally with the main valve element 5, the main valve driving member 6 is fitted for sliding movement in a chamber c defined by a cylindrical wall 8 in the main valve casing 1, the main valve element 5 is disposed on the upstream side of a main valve opening b, a pilot valve A which closes to close the main valve unit when the pressure of a fluid in a downstream passage of the main valve unit rises beyond a predetermined level and a pilot valve B which closes to open the main valve unit when the pressure of the fluid in the downstream passage of the main valve unit drops below a predetermined level are connected by a passage m connected to a main valve driving pressure chamber e formed between the main valve driving member 6 and a main valve top 2, and the pilot valve A and the pilot valve B are arranged in series between an inlet passage a and an outlet passage d to make the main valve driving pressure chamber e function as an operating pressure chamber to drive the main valve unit.
(2) In the automatic-control lift valve device as an automatic constant-flow valve device as shown in FIG. 10, a variable orifice 16 is disposed at an appropriate position in the inlet or the outlet passage of the main valve unit operated by the pilot valves. Pressures in the upstream and the downstream side of the orifice are applied to the secondary pressure chambers k and the spring chambers j of the pilot valve units, respectively, to make the automatic-control lift valve device function as the automatic constant-flow valve.
Although the control valve device of the original invention achieves objects satisfactorily as a constant-pressure valve or a constant-flow valve, and is a high-performance control valve of a simple construction, problems still reside in the same control valve as considered from the viewpoint of actual working condition. When the control valve is used practically, actual working condition often requires the control valve to operate as a constant-pressure valve in a normal operating state and to operate as a constant-flow valve for limiting flow rate when fluid consumption rate in the downstream passage increases excessively. The automatic-control valve device of the original invention is incapable of dealing with such actual working condition alone. If the automatic-control valve device is formed as a constant-pressure valve and the pressure in the downstream passage drops due to excessive increase in fluid consumption rate in the downstream passage, the main valve unit of the automatic-control valve exercises the original function of a constant-pressure valve to compensate for the pressure drop, whereby the function of the automatic-control valve device to maintain the flow rate constant is spoiled. If the automatic-control valve device is formed as a constant-flow valve and the flow rate in the downstream passage decreases, the main valve unit of the automatic-control valve device exercises the original function of a constant-flow valve to compensate for a reduction in the flow rate, whereby the function of the automatic-control valve device to maintain the pressure constant is spoiled.
It has been considered that it is difficult to realize both the function of a constant-pressure valve and that of a constant-flow vale by a simple construction. Therefore, it has been usual to use a constant-pressure valve and a constant-flow valve in combination or to use a valve provided with an expensive sensors and a changeover device for changing the function of a constant-pressure valve for that of a constant-flow valve and vice versa.
Accordingly, it is an object of the present invention to provide a convenient automatic-control valve device solving the foregoing problems residing in the original invention, having all the advantages of the original invention that the fluid does not leak at all when the main valve unit is closed, not provided with any fixed restricting device, such as a needle valve, and hence free from troubles due to clogging attributable to a fixed restricting device, comprising valve units having self-cleaning capability and capable of saving maintenance work, and capable of properly exercising both the function of a constant-pressure valve and that of a constant-flow valve.
Another object of the present invention is to provide a convenient automatic-control valve device capable of easily and simply incorporating a liquid level control unction and an emergency shutoff function therein.